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Evolution

At the feet of dinosaurs

Small lizards that were around at the time of the dinosaurs hide in Brazilian forests

GABRIEL SKUK SUGLIANO/UFAL E MIGUEL TREFAUT RODRIGUES/USPClockwise: C. amazonicus, C. meridionalis and Coleodactylus septentrionalisGABRIEL SKUK SUGLIANO/UFAL E MIGUEL TREFAUT RODRIGUES/USP

Lizards not much bigger than your thumbnail, collected in 16 Brazilian states, in the country’s Northern, Northeastern and Midwestern regions, show that Brazil’s territory shelters living species whose ancestors lived alongside dinosaurs. They also suggest that the country’s biological diversity may be much greater than imagined and that genetic analysis may overturn knowledge that at one point seemed unlikely to ever be contested. The zoologists and geneticists had no idea that they would discover so much just by studying five species of lizards of the Coleodactylus genus from the Amazon region, the semi-arid Caatinga region, and the Atlantic Rainforest.

At first glance the five species look similar, but genetically they are different. DNA analysis carried out at the University of São Paulo (USP) and at the University of California at Berkeley, in the USA, revealed differences even within a single species and indicated that the first examples of the Coleodactylus genus appeared there some 72 million years ago. This result refers to a much earlier time than previous estimates, which indicated 2 million years at most.

“The great dinosaurs must have flattened hundreds of Coleodactylus,” states the zoologist from USP, Miguel Trefaut Rodrigues. He is one of the co-authors, together with Silvia Geurgas, from USP, and Craig Moritz, from Berkeley, of a recent article published in the journal Molecular Phylogenetics and Evolution along with these results. The Coleodactylus, which are the world’s smallest lizards, measuring four centimeters from head to tail, survived in the forests until their descendants, trillions of generations later, made it through to modern times, probably without any bodily modifications during the last 40 million years.

“The time of origin and differentiation between the species that we arrived at throws everything that we knew about Coleodactylus out the window,” admits Rodrigues. The classification used with no problems so far was put forth some 40 years ago by the USP zoologist Paulo Vanzolini and was based on morphological characteristics. To Vanzolini, the Coleodactylus meridionalis, now found in the Atlantic forest and in the Caatinga region, would be a sister-species (very close) of the C. septentrionalis, found in grasslands in the state of Roraima, as well as in Venezuela, Surinam and Guyana. Since these two species lived very far apart, separated by the Amazon Rainforest, which houses a more recent species, C. amazonicus, Vanzolini imagined that one continuous forest may have previously occupied the entire area that now comprises the Amazon region, the Cerrado region, the Caatinga region and the Atlantic Rainforest. The populations of an ancestral species of Coleodactylus would have spread across this gigantic forest and diversified as the vegetation began to change, in response to cyclical variations of temperature.

Genetic analysis has confirmed that the Coleodactylus amazonicus species is radically different from the others – so much so that it represents an evolutionary lineage virtually independent from the other species of Coleodactylus, which must have come into existence shortly after the group’s appearance, about 70 million years ago, at the same time as other genera of this family of South American and Central America began to split off into separate branches. However, from that point on, it is a different story. Genetically, C. meridionalis, found in the Atlantic Rainforest and in forested parts of the Caatinga region, is closer to C. brachystoma, from the Cerrado region, than it is to C. septentrionalis, found in the Amazonian grasslands. In broader terms, geographically closer species – rather than those that are morphologically more similar ? are also more closely related.

The relationship between these lizards is likely to become even more jumbled as biologists link each species with the geographical peculiarities of the environments in which they live. The five species are likely to become at least 17. “There is no one species for the entire Amazon region or the entire Atlantic forest or for the entire Cerrado region,” guarantees Silvia. “For the Amazon region, I would suggest that there are five species, because the molecular data indicates that these are separate evolutionary entities that no longer interbreed.” The molecular studies indicate that most of the species would have a much more restricted geographical distribution, although more than one may possibly share the same territory.

One hypothesis to be tested with the Coleodactylus is whether or not rivers act as barriers to species differentiation. Years ago, Kátia Pellegrino, who is now at the Federal University of São Paulo, together with Rodrigues and some other biologists, demonstrated the validity of that idea with a type of gecko found in the Atlantic Rainforest, the Gymnodactylus darwinii. The populations of this species found north and south of the Doce River (which drains areas of Minas Gerais and Espírito Santo), formerly regarded as being close relatives, have been shown not to be so close after all, as one has 38 chromosomes whereas the other has 40.

One river, various species
Rodrigues, along with his team, previously verified that the São Francisco River isolated populations and favored the appearance of new species, given that in the hot sands on the river’s left and right banks, which are no more than 300 meters apart in the north of the state of Bahia, live lizards that are slightly different from each other. These are the so-called sister-species; as a result of separation, they developed their own evolutionary history, which has only now been explained. This year, comparing DNA sections of ten populations of lizards of the Eurolophosaurus genus, José Carlos Passoni, Maria Lúcia Benozzati and Rodrigues, all from USP, demonstrated in the journal Molecular Phylogenetics and Evolution that these animals must have also appeared earlier than previously imagined, although their origins do not go as far back as those of the Coleodactylus. According to the genetic analysis, one of the species, the Eurolophosaurus divaricatus, a 25-centimer long lizard that lives on the left bank of the São Francisco River, first appeared there 5.5 million years ago. The inhabitants of the river’s opposite bank were deemed to have arrived more recently, with the E. nanuzae having first appeared 3.5 million years ago while the E. amathites appeared at least 1.5 million years ago.

Even so, the estimated origin of these lizards is far earlier than the modest 15 thousand years previously calculated based on geomorphological data. It would have been during this time that the river’s course from the interior to the sea must have shifted as the landscape changed. The ponds on whose banks the lizards used to sunbathe may have disappeared or the river may have incorporated part of the left bank when it started to flow east rather than west. The biologists hope that one day the river’s history may coincide with that of the lizards.

The Project
Systematics and evolution of neotropical herpetofauna (nº 03/10335-8); Type: Thematic Project; Coordinator: Miguel Trefaut Rodrigues – USP; Investment: R$ 900,191.26

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